The present invention relates to air control and distribution devices used in commercial air distribution systems. More specifically, the present invention relates to a flow sensor for a compressible fluid, such as air, having a fluid flow, wherein the flow sensor samples and averages the total pressure profile over the cross section of a duct.
Airflow sensors are used to measure the quantity of air flowing in the ducting of air-conditioning systems to provide control signals to an air terminal. Air terminals are devices of an air-conditioning system that modulate the volume of air delivered to a conditioned space, e.g., of a building, in response to a given thermal load. Examples of various types of air terminals include: variable air volume (VAV) units, bypass terminals, integral diffuser terminals and dual duct terminals.
As the air to be measured flows through the ducting from an upstream direction toward a downstream direction, pressure is generated in all directions in the form of a static pressure. Moreover, on the duct walls perpendicular to the direction of flow, the static pressure is the primary component of pressure. In addition, there is an impact pressure (velocity pressure) on anything that is facing upstream of the direction of flow. The combination of these two forces is called total pressure. Thus, the force on any object in the airflow path that faces the upstream direction has this total pressure acting on it. To derive airflow as a quantity over rate of time, e.g., cubic feet per minute (CFM), from the pressures measured in a duct system, one must subtract the measured static pressure from the measured total pressure to obtain the velocity pressure.
Prior art pitot tube type instruments have been used to perform this task. However, the use of a single pitot tube in the air stream does not provide accurate results. This is because the total pressure varies at different points along the cross section of a duct system, due primarily to frictional drag forces of the air against the duct walls.
Problematically, the traditional pitot tube design provides no amplification of the differential between the static pressure and the total pressure, which is often very small. This is especially problematic in cases of low velocity pressures that are inherent to low CFM, such as when a VAV unit is operating at its minimum rated flow. Consequently, many industrial pressure transducers have difficulty reacting to changes in velocity under low flow conditions when used with such prior art flow meters utilizing a traditional pitot tube design.
Other prior art airflow sensors have been developed which require the averaging of multiple measurements over the cross section of the duct to determine total pressure. One such prior art airflow sensor is disclosed in U.S. Pat. No. 5,481,925 to Woodbury, filed on Sep. 9, 1994 (hereinafter xe2x80x9cWoodburyxe2x80x9d).
Woodbury describes an airflow sensor adapted to be mounted within a flow conduit. The airflow sensor includes a streamlined central hub, at least two pair of upstream airflow sensing tubes and a corresponding number of downstream airflow sensing tubes. The central hub is of air foil shape having an upstream region smoothly transitioning to a downstream region. The upstream airflow sensing tubes are diametrically opposed and extend radially outwardly from the central hub. Each of the upstream airflow sensing tubes including a plurality of radially spaced holes. The holes fluidly connect regions exterior to the tube to an internal flow passage thereof. The holes are spaced from one another such that each hole receives airflow from an equal concentric cross-sectional area of a flow conduit and face upstream to sample the varying total pressure profile along the conduit. The upstream tubes also include a streamlined attachment member for attaching the tubes to an inner wall of the flow conduit. The downstream airflow sensing tubes extend radially outwardly from the central hub a distance shorter than a distance of radial extension of the upstream airflow sensing tubes. Each downstream airflow sensing tube has a single inlet at an end thereof and is circumferentially spaced from a respective adjacent upstream airflow sensing tube in order to measure static pressure within the duct.
However the plurality of upstream facing holes of Woodbury are spaced such that each hole receives airflow from an equal concentric cross sectional area of flow in the duct. This spacing pattern fails to account for the frictional losses at the duct wall. This error can be especially problematic in rectangular ducts.
Moreover, noise is often a problem in prior art flow sensors. Excessive noise levels generated by a flow sensor under normal operating conditions can be a critical factor in a potential customer""s decision to buy. The noise generation issue is of such importance that the Air-Conditioning and Refrigeration Institute (ARI), one of the primary standard making bodies of the refrigeration industry, has developed Standard 880 (herein incorporated by reference). Standard 880 measures and publishes noise levels for air terminals, of which airflow sensors are very often used therein. In order to enhance performance ratings during the standardized test conditions imposed by ARI 880, many airflow sensors, e.g., Woodbury, have a streamlined shape. However, certain streamlined shapes perform better than others for different types of equipment. It is therefore important for a manufacturer to determine the best streamline shape required to optimize noise level performance of its equipment.
Accordingly, there is a need for an improved flow sensor for measuring the flow of a compressible fluid, such as air, in a duct.
The present invention offers advantages and alternatives over the prior art by providing a flow sensor for a compressible fluid, which utilizes radially spaced holes to average the total pressure profile of the fluid flow in a duct. Additionally, the flow sensor utilizes a static tube having a closed distal end and a flow passage therethrough to amplify the differential between the static pressure and the total pressure. Moreover, the flow sensor has a streamlined shape to reduce operating noise levels and minimize pressure drop across the flow sensor.
These and other advantages are accomplished in an exemplary embodiment of the invention by providing a flow sensor for compressible fluid having a fluid flow from an upstream direction toward a downstream direction. The flow sensor comprises a smoothly contoured central hub having a total pressure chamber and a static pressure chamber disposed therein. A pair of wings extends radially outward from opposing sides of the total pressure chamber of the hub. The wings include a plurality of radially spaced holes oriented to generally face the upstream direction and an internal wing passageway connecting the spaced holes to the total pressure chamber. The flow sensor also includes a static tube having a tube wall extending radially outward from the static pressure chamber of the hub from a first attached end to a second distal closed end. A generally upstream facing hole and a generally downstream facing hole are disposed on opposing sides of the tube wall intermediate the first and second ends to define a fluid flow passage therethrough. An internal tube passageway connects the flow passage to the static pressure chamber.
In an alternative embodiment of the invention, the flow passage of the static tube of the flow sensor is shaped to provide a vortex effect which reduces static pressure of the fluid within the static pressure chamber relative to static pressure of the fluid external to the static chamber. By reducing the static pressure, the differential between the static pressure and total pressure is effectively amplified.
In another embodiment of the invention the radially spaced holes on the wings of the flow sensor are substantially spaced per the Log-Tchebycheff rule. By spacing the holes as such, losses due to frictional forces between the walls of a duct and the fluid flow are taken into account.
In another alternative embodiment of the invention the wings and hub of the flow sensor have a substantially elliptical shape to reduce noise levels generated by the flow sensor and minimize pressure drop across the flow sensor.